It is known to purify (or treat) water through a variety of commercially available water treatment devices designed for point-of-use applications in the commercial/residential water treatment market. The most popular devices are based on the following technologies: activated carbon for organic removal: ultraviolet light disinfection: ion exchange for hardness removal (water softening), and membrane desalination such as reverse osmosis (RO) or nanofiltration (NF).
RO membranes have been used for desalination purposes for over two decades now. However, nanofiltration is relatively new in the field of water treatment technology. A NF membrane produces soft water by retaining the hardness creating, divalent ions present in water. A NF membrane allows a high percentage of monovalent ions such as sodium and chloride to pass through, while it retains a high percentage of the divalent ions. It is the monovalent ions that create osmotic pressure that requires the moderate to high pressures necessary to pump water through an RO membrane. Therefore, nanofilter membranes require much less pressure to pump water through the membrane because hydraulic driving force does not have to overcome the effect of osmotic pressure from monovalent ions. Generally speaking, RO membranes used for residential and commercial water treatment applications remove all dissolved solids by approximately 98%. while nanofilter membranes remove divalent ions (hardness components: calcium and magnesium) by approximately 90% and monovalent ions (sodium chloride) by approximately 50%.
Desalination devices that use membrane elements (for example: RO or NF) always create two streams of water as the water exits the element: desalinated product water (which has passed through the membrane), and a waste brine (that has flowed across the membrane surface). This waste brine stream is necessary to flush salts and minerals away from the membrane to prevent them from accumulating and fouling the membrane surface. If a buildup of salts and minerals in the feed-water to a membrane occurs continuously, dissolved substances can precipitate and form a solid film fouling the surface of the membrane. In addition, colloidal and particulate contaminants can also adhere to the membrane surface and cause fouling. With many water-borne contaminants, if a membrane becomes irreversibly scaled, or fouled, it can't be cleaned and must be replaced. This characteristic of membrane processes poses a significant problem in reducing waste effluent especially in point of use (POU) water treatment systems that are typically compact and built as economically as possible.
Ion exchange devices are also used to soften so called "hard water". The problem with ion exchange water softening systems is that they remove the hardness components of water (calcium and magnesium ions) by exchanging them for sodium ions in order to create what is called "soft water". When regeneration of the ion exchange media takes place, a concentrated water stream of sodium, chloride, calcium and magnesium ions goes into the sewer system creating an environmental waste disposal problem.
In a conventional residential or home RO water treatment system, the standard ratio of water wasted to water produced is approximately 5 to 1, or 5 gallons wasted for each gallon produced. However, this is only if the product water goes to atmospheric pressure after exiting the RO module. The ratio is routinely 10 to 1, and can be much higher, in systems containing an air captive storage tank to store RO product water. This form of tank exerts a back pressure that restricts water flux through the RO membrane as the tank is filling (while waste brine continues to flow freely to drain). Therefore, an appreciable amount of water is normally wasted with a conventional home RO system.